1. Technical Field
This disclosure relates to a calibration technique for calculating installation parameters of a camera on the basis of images captured by the camera installed on a movable body.
2. Description of the Related Art
In recent years, a technique has been spread where a camera installed on a vehicle is used to measure distances between the vehicle and objects in front of or to the rear of the vehicle and, when needed, the driver is alerted by an alarm. In order to accurately measure the distance between the vehicle and the object, it is necessary to calculate (calibrate) an installation position and an installation angle of a camera with respect to the vehicle.
In general, as a calibration technique for calculating an installation angle of a camera with respect to the vehicle, a technique utilizing a point (a vanishing point) at which straight lines which are parallel in real space intersect with each other in image space is known.
In a calibration technique utilizing a vanishing point, the installation angle of the camera is calculated using at least two vanishing points: a vanishing point (a deep vanishing point) which is calculated using lines parallel to a travelling direction of the vehicle and a vanishing point which is calculated using other lines parallel to another direction.
In the following description, the installation angle of the camera with respect to the vehicle is referred to as an installation parameter of the camera.
With regard to a calibration technique utilizing a vanishing point to calculate an installation parameter of the camera, there is a technique where two pairs of parallel lines are extracted using a road marking of which graphical features are stored in advance, and then two vanishing points are calculated from the extracted two pairs of parallel lines (see, for example, Japanese Unexamined Patent Application Publication No. 2008-11174). In addition, there is also a technique where a plurality of images are successively captured and accumulated, then an edge is extracted from the accumulated image, whereby a set of lines parallel to the travelling direction of a vehicle is extracted and a deep vanishing point is calculated using the extracted parallel lines (see, for example, International Publication No. 2010/146695).
In the technique described in the above-mentioned Japanese Unexamined Patent Application Publication No. 2008-11174, in order to calculate the vanishing point, graphical features of the road marking need to be stored in advance. Further, if a vehicle does not encounter a road marking which coincides with any of the road markings of which features are stored, it would be impossible to calculate an installation parameter of a camera.
In addition, in the technique described in International Publication No. 2010/146695, although a deep vanishing point can be calculated, another vanishing point cannot be calculated; therefore, an installation parameter of a camera cannot be calculated with the technique described in International Publication No. 2010/146695 alone.